A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, the size of features that can be imaged onto the substrate is limited by the wavelength of the radiation used to apply the desired pattern onto the substrate. To produce intergrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation in the range of 5 to 20 nm, especially around 13.5 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources include, for example, laser produced plasma sources, and discharge plasma sources or a synchrotron radiation from electron storage rings.
Apparatus using discharge plasma sources are described in: W. Partlo, I. Fomenkov, R. Oliver, D. Birx, “Development of an EUV (13.5 nm) Light Source Employing a Dense Plasma Focus in Lithium Vapor”, Proc. SPIE 3997, pp. 136–156 (2000); M. W. McGeoch, Tower Scaling of a Z-pinch Extreme Ultraviolet Source”, Proc. SPIE 3997, pp. 861–866 (2000); W. T. Silfvast, M. Klosner, G. Shimkaveg, H. Bender, G. Kubiak, N. Fomaciari, “High-Power Plasma Discharge Source at 13.5 and 11.4 nm for EUV lithography”, Proc. SPIE 3676, pp. 272–275 (1999); and K. Bergmann et al., “Highly Repetitive, Extreme Ultraviolet Radiation Source Based on a Gas-Discharge Plasma”, Applied Optics, Vol. 38, pp. 5413–5417 (1999).
EUV radiation sources, such as discharge plasma radiation sources referred to above, may require the use of a rather high partial pressure of a gas or vapor to emit EUV radiation. In a discharge plasma source, for example, a discharge is created in between electrodes, and a resulting partially ionized plasma may subsequently be caused to collapse to yield a very hot plasma that emits radiation in the EUV range. The very hot plasma is quite often created in Xe, since a Xe plasma radiates in the Extreme UV (EUV) range around 13.5 nm. For an efficient EUV production, a typical pressure of 0.1 mbar is needed near the electrodes to the radiation source. A drawback of having such a rather high Xe pressure is that Xe gas absorbs EUV radiation. For example, 0.1 mbar Xe transmits over 1 m only 0.3% EUV radiation having a wavelength of 13.5 nm. It is therefore required to confine the rather high Xe pressure to a limited region around the source. To achieve this, the source may be contained in its own vacuum chamber that is separated by a chamber wall from a subsequent vacuum chamber in which the collector mirror and illumination optics may be obtained.
The vacuum wall may be made transparent to EUV radiation by a number of apertures in the wall, provided by a channel array or so-called foil trap, such as described in European Patent application number EP-A-1 057 079, which is incorporated herein by reference. In order to reduce the number of particles propagating along the optical axis, a channel array or “foil trap” has been proposed in EP-A-1 223 468 and EP-A-1 057 079. This foil trap consists of a channel-like structure that includes lamella shaped walls close together in order to form a flow resistance, but not too close so as to let the radiation pass without obstructing it. This foil trap is incorporated herein by reference.
The contamination of the optical components of the lithography apparatus by relatively heavy, micron-sized particles or smaller particles having a relatively low velocity, which are emitted by the EUV source and which pass the channel array in the vacuum wall of the source poses a serious problem, as this contamination results in degradation of the optical components and considerably increases the operational costs of an EUV lithographic projection apparatus.
It has been proposed to provide a rotating foil trap, as disclosed in EP 1 274 287 A1, which is incorporated herein by reference. This foil trap includes platelets which extend radially from a rotation axis. In other words, the rotation axis is parallel to all the platelets. In operation, gas molecules or other contaminating particles that hit the platelets are, on average, directed into a direction of the motion of the platelets.
The surface area of the platelets is generally proportional to the length of the rotational axis around which the platelets rotate.
EP 1 391 785 proposes a contaminant trapping system that includes two foil traps which are aligned such that radiation may pass through both foil traps. The foil traps may rotate with respect to each other so that particles which still manage to pass through the first foil trap may be trapped by the second foil trap.